How does the compositor interact with the 3d viewer? (Chroma keying movies as planes)
The 3D Viewport is the place where you can manipulate your models, and modify the way they look. You can use different shading techniques to see how your objects react in different scenarios.
It has a few "views" :


Wireframe mode - Where you can see only the edges of your mesh, and can see through the faces
Solid mode - The shading is simplified to help you focus on the geometry for example
Lookdev mode - Where you can see an approximation of the final textured result thanks to the real-time render engine Eeevee
Rendered mode - Where you see the scene as it will be rendered when you choose to output an image out of it. It is supposed to be slower so it is not recommended to use this mode when modifying your geometry


When you are happy with your rendered look, you can choose to render an image or a video of your scene. You need to add a camera and position it so that the parts you want to be in the final images are inside its field of view. You can see what will be rendered by pressing  numpad 0 . When you are content with it, you then can render the image by pressing  F12  for a still image or CTRL  +  F12  for an animation (or go the render menu on top of the blender window).

Then the image is rendered, and you can add post-processing compositing effects, for example changing certain colors, adding flares or adding a vignette effect. This is a one-way process and can only be applied to rendered images. It cannot be used to then be applied back into the 3D viewport.

If you want a real-time chroma key you will have to build a custom shader for that, because the compositor will only affect the output of your rendered images.